Repeated freeze–thaw cycles and their effects on biological processes in two arctic ecosystem types

Abstract Mesocosms from two subarctic heaths from high and low altitudes, respectively, and differing in the composition of vegetation were experimentally subjected to 18 diurnal freeze–thaw cycles fluctuating between day temperatures of +2 °C and night temperatures of –4 °C. Mesocosm respiration was consistently lower and higher in cycled mesocosms compared to mesocosms kept unfrozen (+2 °C) and frozen (–4 °C), respectively. When extrapolated to the entire non-growing season, our data suggest a non-growing season carbon (C) loss in the order of 20–30 g C m−2, which is substantial compared with a likely annual incorporation of plant biomass C of 150–200 g C m−2 per year. The results support the hypothesis that winter carbon fluxes must be included in annual carbon budgets for arctic ecosystems. Microbial biomass C decreased only in cycled mesocosms, indicating that while microbial activity is mainly controlled by temperature, microbial biomass is strongly affected by temperature fluctuations around the freezing point. In contrast to microbial C, microbial biomass nitrogen (N) mass did not change in cycled mesocosms, leading to decreased microbial C to N ratio. Second, soil inorganic N concentration declined in the soils subjected to the cycles, showing that microbes surviving the freeze–thaw cycles had a high potential for sequestering soil N and N that presumably was released from dead microbes. The two sets of mesocosms responded very similarly to the freeze–thaw cycles, indicating that the observed responses to multiple diurnal freeze–thaw cycles are common for both ecosystem types despite the differences in plant species composition and climate.